There has been known a method for assaying an endotoxin by using limulus amoebocyte lysate components (hereinafter referred to as the limulus test), which has been frequently employed in the examination on the contamination of drugs and water and in clinical tests. The limulus test is suited for the detection of a trace amount of endotoxin in a biological specimen because of its high detection sensitivity.
A biological specimen contains an endotoxin adsorbed by, for example, proteins. It is therefore required prior to the limulus test to perform a pretreatment of the specimen so that the endotoxin could be efficiently liberated from the proteins. It is known to pretreat a biological specimen (for example, plasma, serum) containing granulocytes by treating a sample with an acid, having a pKa of 3 or below at 25.degree. C., at a pH value of 3 or below, removing the denatured precipitate by centrifugation, collecting the supernatant and neutralizing it with an alkali to serve as a sample solution (JP-B-63-55671; the term "JP-B" as used herein means an "examined Japanese patent publication"). However this method is disadvantageous in that the separation procedure is complicated and requires a long period of time for completing the whole procedure and that the specimen would be possibly contaminated.